Staphylococcus aureus (S. aureus) is a ubiquitous gram positive bacterium that can colonize the nares and skin of humans without causing disease. Approximately one third of the human population is colonized with S. aureus making it difficult to avoid transmittance. The bacteria can cause a wide variety of disease from mild skin infections to more serious diseases such as bacteremia and endocarditis. The patient populations most at risk are dialysis patients, patients with ventriculoperitoneal shunts, patients at risk of infective endocarditis, patients who are immunocompromised, and residents of nursing homes. In healthcare settings it is the main pathogen responsible for infections of the skin and soft tissues, as well as for those associated with medical procedures and indwelling devices such as catheters. Since catheter- and device-related infections remain the most significant cause of morbidity, prolonged length of stay and increased cost in affected patients, S. aureus infections are of concern. S. aureus has developed resistance to multiple antibiotics and has a methicillin-resistant variant (MRSA) which is becoming widespread in the community and nosocomial environments. This is leading to increased incidences of infection in both the hospital and community settings. With reduced treatment options available, alternative approaches are required.
S. aureus is an important cause of serious infections in both hospitals and community. Unfortunately, this pathogen has the ability to quickly respond to each new antibiotic and has been particularly efficient at developing resistance mechanisms. In 1942, two years after the introduction of penicillin for medical use, the first penicillin-resistant S. aureus isolate was observed in a hospital. Since 1960, around 80% of all S. aureus strains are resistant to penicillin. In 1961, two years after the introduction of methicillin, a penicillinase-resistant penicillin, S. aureus, developed methicillin-resistance due to the acquisition of the mecA gene. During the last several decades, various methicillin-resistant S. aureus (MRSA) clones disseminated worldwide become a global health threat and a significant challenge for healthcare systems. Since the widespread emergency of MRSA, vancomycin has represented the cornerstone of therapy for MRSA infections. Over the last decade, strains that are not susceptible to vancomycin have occurred, showing either intermediate resistance (VISA) or, worse, full resistance to this antibiotic (VRSA).
Currently, the rapid emergence of bacteria resistant to commonly used antibiotics has become a serious problem and one of the major challenges for the healthcare systems worldwide. Antibiotic resistant infections are associated with higher treatment cost, longer hospital stay and a 1.3 to 2-fold increase in mortality. The resistant bacteria also spread to community and become broader infection-control problems, since some community-associated resistant strains are more virulent due to the production of virulent factors like toxins.
Despite the urgent need for effective agents to overcome the bacterial resistance, the antibiotic drug discovery and development has slowed considerably in recent years. Traditional approaches and the newer genomic mining approaches have not yielded novel classes of antibacterial compounds. An alternative strategy is to improve analogues of existing classes of antibacterial drugs by modifying the action sites or combining with other compounds to improve the potency and minimize the resistance.
Methicillin resistant bacteria typically exhibit resistance to all, or at least most, β-lactam antibiotics, including penicillins, cephalosporins and carbapenems. β-lactam antibiotics target the transpeptidase activity of penicillin-binding proteins (PBPs) involved in cell wall biosynthesis. The peptidoglycan layer in the bacterial cell wall is a crystallattice structure formed from linear chains of two alternating amino sugars, N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). Each MurNac is attached to a short amino acid chain, containing L-alanine, D-glutamic acid, L-lysine, and D-alanyl-D-alanine in the case of S. aureus. Cross-linking between amino acids in different linear amino sugar occurs with the help of the enzyme transpeptidases and results in a 3-dimensional structure that is strong and rigid. β-lactams contain the highly reactive CO—N bond in the β-lactam ring, which lie in exactly the same position as the CO—N bond in D-alanyl-D-alanine, resulting in nearly identical conformation of the terminal portion of the peptidoglycan peptide chain, which is the target of transpeptidation. The binding of β-lactams to transpeptidase enzymes (also known as penicillin-binding proteins, PBPs) results in acylation of a serine residue in the active site of PBP, this irreversible reaction inactivates the enzyme and prevents the final cross linking (transpeptidation) of the nascent peptidoglycan layer, disrupting cell wall synthesis.
Methicillin sensitive S. aureus (MSSA) express four naïve penicillin-binding enzymes (PBP1, PBP2, PBP3, and PBP4), and their activities are specifically prevented by the covalent binding of β-lactam antibiotics to their active sites. The MRSA isolates acquired the mecA gene, coding for a novel 78 KDa penicillin-binding protein 2a (PBP2a). The crystal structure of PBP2a reveals it to have a closed active site, and the interactions of PBP2a with peptidoglycan at an allosteric site trigger a conformational change that leads to accessibility to the active site. PBP2a is not inhibited in the presence of β-lactams due to the blockage of access to the active site, and is able to take over the peptidoglycan biosynthesis from the naïve PBPs to perform the critical cell wall cross-linking reaction.
Vancomycin is a glycopeptide antibiotic that is able to form hydrogen bond interactions with the terminal D-alanyl-D-alanine moieties of the peptidoglycan (murein) monomer. The binding of vancomycin to the D-Ala-D-Ala prevents cell wall synthesis in two ways. It can completely or partially inhibit the peptidoglycan polymerization if it binds to murein monomers located in the cytoplasmic membrane. It also can target the D-Ala-D-Ala residues in the completed peptidoglycan layers or on the nascent peptidoglycan chain to prevent the cross-linking of the backbone polymers.
VISA and VRSA have emerged almost exclusively from MRSA, with few exceptions involving strains with hetero-resistance. Vancomycin resistance does not develop step-wise and VRSA does not progress from VISA, since VISA and VRSA have completely different resistance mechanisms.
The intermediate resistance in VISA has been associated to the presence of a thickened cell wall. The wall is rich in peptidoglycan chains that are not cross-linked and display free terminal D-Ala-D-Ala residues, which act as decoy targets, blocking vancomycin in the external layer of the cell wall and diverting the antibiotic from reaching its true target-peptidoglycan precursors at cytoplasmic membrane level. The cell wall is clogged by the trapped vancomycin and this further block the antibiotic penetration. No characteristic genetic trait has been tightly associated with VISA resistance.
Different from VISA, VRSA strains have acquired the complete genetic apparatus for glycopeptides resistance from vancomycin-resistant enterococci (VRE). VRSA strains acquired the vanA operon that confers high level resistance to glycopeptides, vancomycin and teicoplanin. The vanA operon contains an assembly of genes that encode the synthesis of modified peptidoglycan precursors containing a terminal D-Ala-D-Lac instead of D-Ala-D-Ala and the elimination of the susceptible wild-type targets. The D-alanyl-D-lactate variation results in the loss of one hydrogen-bonding interaction (4, as opposed to 5 for D-alanyl-D-alanine) possible between vancomycin and the peptide, which results in a 1000-fold decrease in affinity. The resistance mechanism is under the regulation of a two-component signal transduction system (gene vanS and vanR), which activates only in the presence of vancomycin.
Therefore, there is an unmet need for effective treatment and/or prevention of S. aureus associated infections that are resistance to current antibiotics, including MRSA, VISA, and VRSA.